Metal fusion bonding – Process – Repairing – restoring – or renewing product for reuse
Reexamination Certificate
1998-09-21
2001-07-31
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Repairing, restoring, or renewing product for reuse
C228S175000, C228S184000, C422S241000, C220S003940
Reexamination Certificate
active
06267289
ABSTRACT:
The present invention relates to a method for the safety of pressure equipment in contact with corrosive fluids and to the modified equipment thus obtained.
More specifically, the present invention relates to a method for the safety of equipment normally operating under pressure, which is in contact with corrosive fluids and therefore comprises anticorrosive lining overlying the sealing structure (pressure-resistant body).
Typical equipment of this kind is that which is present in many industrial chemical plants, such as, for example, reactors, heat-exchangers, condensers and evaporators, whose operating conditions comprise pressures of between 50 and 1000 bars and temperatures of between 100 and 500° C., in contact with acid, basic or generally saline fluids having high corrosive potential especially with respect to carbon or low-alloy steel which is the material normally selected for the sealing of equipment.
Typical processes which require the use of high pressure equipment in contact with corrosive fluids are, for example, those for the production of urea by direct synthesis starting from ammonia and carbon dioxide. In these processes, ammonia generally in excess and carbon dioxide are reacted in one or more reactors, at pressures usually of between 100 and 250 bars and temperatures of between 150 and 240° C., obtaining a mixture at the outlet consisting of a water solution of urea, ammonium carbamate not transformed into urea and the excess ammonia used in the synthesis. The reaction mixture is purified of the ammonium carbamate contained therein by its decomposition in decomposers operating, in succession, at gradually decreasing pressures. In most of the existing processes, the first of these decomposers operates at pressures which are basically equal to the synthesis pressure or slightly lower, and normally uses “stripping” agents to decompose the ammonium carbamate with the contemporaneous removal of the decomposition products. The “stripping” agents can be inert gases, or ammonia or carbon dioxide, or mixtures of inert gases with ammonia and/or carbon dioxide, the “stripping” also being possible by using the excess ammonia dissolved in the mixture coming from the reactor (auto-stripping) without supplying therefore any external agent.
The decomposition products of ammonium carbamate (NH
3
and CO
2
) together with the possible “stripping” agents, excluding inert gases, are normally condensed in suitable condensers obtaining a liquid mixture comprising water, ammonia and ammonium carbamate, which is recycled to the synthesis reactor. In plants which are technologically more advanced, at least one condensation step is carried out at pressures which are basically equal to or slightly lower than those of the reactor.
As a reference, it is possible to cite, among the many existing ones, patents U.S. Pat. Nos. 3,886,210, 4,314,077, 4,137,262, and published European patent application 504.966, which describe processes for the production of urea with the above characteristics. A wide range of processes mainly used for the production of urea is provided in “Encyclopedia of Chemical Technology”, 3° Edition (1983), Vol.23, pages 548-574, John Wiley & Sons Ed.
The most critical steps in the process are those in which the ammonium carbamate is at its highest concentration and highest temperature, and therefore in the above processes, these steps coincide with the reactor and subsequent equipment for the decomposition (or stripping) and condensation of the ammonium carbamate operating under analogous or similar conditions to those of the reactor. The problem to be solved in this equipment is that of the corrosion and/or erosion caused by the ammonium carbamate, ammonia and carbon dioxide which act as highly corrosive agents, especially in the presence of water, at the high temperatures and pressures necessary for the synthesis of urea.
Various solutions to the problems of corrosion of the type described above have been proposed, many of which have been applied in existing industrial plants. Numerous metals and alloys are in fact known which are capable of resisting for sufficiently long periods, in various cases, to potentially corrosive conditions which are created inside industrial chemical equipment. Among these lead, titanium, zirconium, tantalium and several stainless steels such as, for example, AISI 316L (urea grade), INOX 25/22/2 Cr/Ni/Mo steel, austenitic-ferritic steels, etc., can be mentioned. However, for economic reasons, the above type of equipment is not normally entirely constructed with these corrosion-resistant alloys or metals. Usually hollow bodies, containers or columns are produced in normal carbon or low-alloy steel, possibly with several layers, having a thickness varying from 20 to 400 mm, depending on the geometry and the pressure to be sustained (pressure-resistant body), whose surface in contact with the corrosive or erosive fluids is uniformly covered with an anticorrosive metal lining from 2 to 30 mm thick.
In the above plant equipment or units, the anti-corrosive lining is produced by the assembly and welding of numerous elements appropriately shaped to adhere as much as possible to the form of the pressure-resistant body, in order to create, at the end, a structure hermetically-sealed against the high operating pressure. The different junctions and weldings carried out for this purpose frequently require the use of particular techniques depending on the geometry and nature of the parts to be joined.
Whereas stainless steel can be welded to the underlying “pressure-resistant body” made of carbon steel, but has a higher thermal expansion coefficient which favours, during operation, the creation of fractures along the welding line, titanium cannot be welded to steel and in any case has analogous fracture problems in the weldings as it has an expansion coefficient which is much lower than carbon steel.
For this reason resort is made to techniques which often require complex equipment and operating procedures. In certain cases the lining is effected by welding deposit instead of plates welded to each other and onto the pressure-resistant body. In other cases, especially with materials which cannot be welded to each other, it is necessary to “explode” the lining onto the pressure-resistant body to be sure of obtaining a satisfactory support.
A certain number of “weep-holes” are however applied to all the above equipment for the detection of possible losses of anticorrosion lining.
A weep-hole normally consists of a small pipe 5-30 mm in diameter made of a material which is resistant to corrosion and is inserted in the pressure-resistant body until it reaches the contact point between the latter and the lining in corrosion-resistant alloy or metal. If there is a loss of lining, owing to the high pressure, the internal fluid, which is corrosive, immediately spreads to the interstitial area between the lining and the pressure-resistant body and, if not detected, causes rapid corrosion of the carbon steel of which the latter is made. The presence of weep-holes enables these losses to be detected. For this purpose all interstitial areas underneath the anticorrosion lining must communicate with at least one weep-hole. The number of weep-holes is usually from 2 to 4 for each ferrule, therefore, for example, in a reactor of average dimensions, having a surface expansion of about 100 m
2
, there are normally about 20 weep-holes.
The above equipment also has, normally in the upper part, at least one circular opening, called “man-hole”, which allows access to operators and equipment for inspections and minor internal repairs. These
10
openings usually have diameters of between 45 and 60 cm and at the most allow the passage of objects having a section within these dimensions.
In spite of the above measures, it is generally known that the welding lines and points of the protective “lining” form a weak point in the structure of chemical equipment. In fact microfractures can be found during operation for the above reasons of different thermal expansion between the ma
Dunn Tom
Edmondson L. E
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Snamprogetti S.p.A.
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